Title
Consider/Discuss/Act on a Request for a Variance to the Detention Requirements of the Stormwater Management Ordinance for the Oak Hollow Office Park Development
Summary
MEETING DATE: August 20, 2013
DEPARTMENT: Engineering
CONTACT: W. Kyle Odom, CFM, RS, Environmental Engineering Manager
Daniel Still, PE, CFM, Civil Engineer
RECOMMENDED CITY COUNCIL ACTION:
· City staff recommends approval the variance request regarding the 10-year storm.
· City staff recommends denial of the request regarding the 1-year storm.
ITEM SUMMARY:
· The developer of the Oak Hollow Office Park has requested that the City Council grant a variance to the detention requirements of the Stormwater Management Ordinance.
BACKGROUND INFORMATION:
· In 2006, the City Council directed City staff to amend the Stormwater Management Ordinance to address erosion in McKinney's creeks and streams. The 2006 amendments include the following:
o Runoff from the 1-year, 24-hour storm shall be detained and released in not less than 12 hours.
o The post-development 10-year peak discharges shall not exceed the pre-development 10-year peak discharges.
o If a developer elects not to detain as indicated above, then the channel stability of the impacted stream must be evaluated downstream to the nearest major receiving stream and, if necessary, channel stabilization measures must be constructed or financially guaranteed.
o Also, detention of storms larger than the 10-year storm (i.e. the 100-year storm) are not advisable nor required if it is determined that such detention would create a coincidental peak (create an increase in the creek's flow rate for the design storm).
· The developer's consultant provided a drainage study of the creek upstream of SH 5 demonstrating that:
o On-site detention will slightly increase the stream 100-year peak flow at SH 5 due to coincidental peaks.
o On-site detention will also slightly increase the stream 10-year peak flow at SH 5 due to coincidental peaks.
o On-site detention will slightly reduce the stream 1-year peak flow at SH 5.
· The developer asserts that, because the benefit of detention is so small during one design storm; and actually detrimental to the other design storms, the detention requirement should be waived.
· It is the position of City staff that, although the non-detained release from each individual site may seem insignificant, in aggregate the non-detained release from all undeveloped sites significantly increases the 1-year stream peak flow, which, in turn, increases downstream erosion.
· Staff evaluated the impact on the applicant's basin if all currently undeveloped properties were allowed to develop under the variance provisions being requested. The significant (74%) increase in flow rate is shown below.
· The developer further asserts that the cost of a downstream evaluation is overly burdensome and, therefore, should not be required. For reference, the nearest major receiving stream for this property is the East Fork of the Trinity River, which is more than a mile away.
· Staff acknowledges that detention at the subject property will cause the 100-year peak flow to increase. But staff maintains that, due to (1) the infrequency of the 100-year storm and (2) the fact that there is downstream capacity to accommodate the minimal increase, downstream capacity issues are not anticipated as a result of detention at this site. Also, the ordinance specifically states that the coincidental peak issue only pertains to storm events greater than the 10-year storm (i.e. the 100-year storm).
· Staff recommends approval of the variance request regarding the 10-year storm as it has been demonstrated that, due to coincidental peaks, detention of the 10-year storm would have a detrimental downstream impact.
· Staff recommends denial of the variance request regarding the 1-year storm based on the following:
o The developer has not performed a downstream channel stability evaluation as required to determine the effects of the proposed development without detention.
o The study that was submitted shows detention will provide a benefit, however slight, to future channel stability.
o When considering detention for the remaining undeveloped portions of the watershed, each slight benefit translates to a significant benefit overall for the property owners along the creek who would otherwise experience additional erosion.
o The downstream creek will experience more erosion if all similar sites are allowed to develop without detention of the 1-year storm.
o Granting this variance will set a precedent in McKinney that will cause significant deterioration of McKinney's natural streams.
o The applicant has not met all three of the required criteria listed in Section 130-268 (b) (2) of the Stormwater Management ordinance:
- A showing of good and sufficient cause;
- A determination that failure to grant the variance would result in exceptional hardship to the applicant, including an exceptional hardship created by the applicability of the effective date to the application for a permit; and
- A determination that the granting of a variance will not result in additional threats to public safety or extraordinary public expense, create nuisances, cause fraud on or victimization of the public or conflict with existing local laws.
FINANCIAL SUMMARY:
· N/A
BOARD OR COMMISSION RECOMMENDATION:
· N/A